DavDroid crashes when Android Calendar provider is deactivated

I added my Posteo CalDav Account (which should work, the posteo guys confirmed it).
In the wizzard I was able to see my Calendar and my Adressbook. If I try to finish the wizard by clicking the tick I get an android message “davdroid has been closed”. The adressbook is now shown in my accounts but not the calendar.

I uninstalled xprivacy and the xposed framework an reinstalled DAVdroid as you said. The problem remains.

Ok. Unfortunately, I don’t have further ideas why resolver.acquireContentProviderClient(CalendarContract.AUTHORITY) might return null… According to the docs, that would mean “there isn’t one” (a CalendarContract.AUTHORITY, i.e. a calendar provider). So I should improve DAVdroid that it logs an error message in this case (instead of crashing), but that wouldn’t solve your problem.

Is there anything else that is different on your device in comparison to a “normal” Android device?

No the rest should be “normal”.
I have no google account added but I guess this is not linked to my problem.
If I add the account, I can see the calendar and the contacts but as I sad afterwards it crashes.